1. Field of the Invention
The present invention relates to a zoom lens suitable to be mounted on an electronic still camera or a camera module for a mobile terminal using an imaging device such as a CCD (Charge-Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor) or the like.
2. Description of Related Art
There has been heretofore developed an electronic imaging apparatus such as an electronic still camera (also referred to as “digital still camera”), an electronic video camera or a camera module for a mobile terminal using an imaging device such as a CCD, a CMOS or the like. In recent years, with the popularization of personal computers over general homes, image information of photographed scenes, persons, etc. can be inputted into personal computers so as to be processed and distributed easily. Thus, the market for the electronic imaging apparatuses has been expanding rapidly.
In these circumstances, imaging lenses (particularly zoom lenses) to be mounted on electronic imaging apparatuses have been intensely demanded to have a good balance among performance, cost and size. As the zoom lens in the background art, there have been known zoom lenses disclosed in JP-A-2003-177315 and JP-A-2005-004020. Each of the zoom lenses disclosed in JP-A-2003-177315 and JP-A-2005-004020 has a four-lens configuration. Specifically, the zoom lens disclosed in JP-A-2003-177315 has a first group constituted by a negative single lens, a second group having a two-lens configuration of a positive lens and a negative lens, and a third group constituted by a positive single lens. In zooming, the first group and the second group can be moved while the third group is fixed. On the other hand, the zoom lens disclosed in JP-A-2005-004020 has a first negative lens group having one positive lens and one negative lens, and a second positive lens group having one positive lens and one negative lens, which lens groups are disposed in this order from the object side. In zooming, the mutual distance between the first lens group and the second lens group is varied.
As a system having a structure similar to those of the imaging zoom lenses, an objective lens system may be mounted on a real image system optical zoom finder (hereinafter referred to as “finder” simply) (for example, see JP-A-9-211547). The finder disclosed in JP-A-9-211547 has an objective lens system and an ocular lens system each having a four-lens configuration with a light shielding unit in a position. Thus, the finder can be made compact and shielded from harmful beams of light causing flare. In such a finder, generally, the objective lens system has a zoom configuration, an erect prism system is provided between the objective lens system and the ocular lens system. The erect prism system is made of not glass but plastic in view from reduction in cost. Thus, in the erect prism system, a reflecting coat is often absent from its reflecting surface. Therefore, the objective lens system is designed so that a beam of light incident on the erect prism system is telecentric.
However, the objective lens system in the finder as disclosed in JP-A-9-211547 is not provided with a fixed aperture stop. This is because the pupil diameter of an observer itself using the finder serves as a fixed aperture stop. In addition, a beam of light leaving the objective lens system and entering the erect prism system is telecentric. The position and the aperture of each lens in the objective lens system are generally varied in zooming. On the other hand, an imaging zoom lens needs an aperture stop. Therefore, if the objective lens system of the finder is used as an imaging zoom lens as it is, the imaging performance will be affected by a difference in height of incidence of the beam. Further, the erect prism system is disposed in the finder as described above. It is therefore necessary to secure a comparatively large back-focal distance corresponding to the erect prism system. Thus, the finder tends to lack compactness. For example, in the finder disclosed in JP-A-9-211547, the ratio of the back-focal distance to the screen diagonal size on the imaging plane of the objective lens system is 2.12 (on an air basis) when it is estimated from the incident view angle. Further, such a finder includes many parts depending on the adjustability of the observer's eye. When the objective lens system serves as an imaging zoom lens, the objective lens system cannot be regarded as sufficient in terms of performance about distortional aberration, spherical aberration, chromatic aberration, etc. In JP-A-9-211547, a highly dispersed material having an Abbe number of 30.0 is used as a constituent material of the first lens group L1. This is because the pupil corresponding to a fixed aperture stop is located on the side of the eyepiece system.
The zoom lens disclosed in JP-A-2003-177315 is a zoom lens for an electronic imaging device to be mounted on a camera module mainly for a mobile terminal. The zoom lens is short in full length and compact, but the back-focal distance is too short to secure flange-focal distance. Further, the exit angle in the wide-angle end is about 20°, which is too large to serve as a general-purpose zoom lens for an imaging device. On the other hand, in the zoom lens disclosed in JP-A-2005-004020, a positive lens and a negative lens are paired in each of the first and second lens groups. It is therefore possible to suppress various aberrations occurring inside each of the first and second lens groups. On the contrary, however, the refracting power of each lens group cannot be made very high. Therefore, if the zoom ratio is to be increased to about three times while good optical performance is kept, the moving distance of each lens group will increase. As a result, the full length of the zoom lens will increase, or the lens diameter will be increased.